iwlwifi: mvm: cleanup references to aggregation count limit

Currently the code is mixing defines and is inconsistent.
When enabling a queue, we usually configure the scheduler
with IWL_FRAME_LIMIT - 64.
When sending to firmware the rate scaling, we limit aggregation
to LINK_QUAL_AGG_FRAME_LIMIT_DEF - 63, due to a scheduler bug.
Given that, clean up the following:
- Fix a stray queue enablement with LINK_QUAL_AGG_FRAME_LIMIT_DEF.
- Change the comparison that tests if queue needs to be reconfigured
  to be compared directly to how it was configured.
  This also saves the redundant round down of the buffer size just
  for the sake of comparing it, making the code more readable.
- Better document gen2 logic

Signed-off-by: Sara Sharon <sara.sharon@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>